Blood coagulation is a complex chemical and physical reaction that occurs when blood (herein, “blood” shall mean whole blood, citrated blood, platelet concentrate or plasma, unless otherwise specified) comes into contact with an activating agent, such as an activating surface or an activating reagent. In accordance with one simplified conceptual view, the whole blood coagulation process can be generally viewed as three activities: platelet adhesion, platelet aggregation, and formation of a fibrin clot. In vivo, platelets flow through the blood vessels in an inactivated state because the blood vessel lining, the endothelium, prevents activation of platelets. When a blood vessel is damaged, however, the endothelium loses its integrity and platelets are activated by contact with tissue underlying the damaged site. Activation of the platelets causes them to become “sticky” and adhere together. Additional platelets then adhere to the activated platelets and also become activated. This process continues until a platelet “plug” is formed. This platelet plug then serves as a matrix upon which blood clotting proceeds.
If the chemical balance of the blood is suitable, thrombin is then produced that causes fibrinogen to convert to fibrin, which forms the major portion of the clot mass. During clotting, additional platelets are activated and trapped in the forming clot, contributing to clot formation. As clotting proceeds, polymerization and cross-linking of fibrin results in the permanent clot. Thus, platelet activation plays a very important function in blood coagulation.
The clinical assessment of clotting function has long been recognized to be important in the management of surgical patients. Preoperatively, the assessment of the clotting function of the patient's blood is utilized as a predictor of risk of patient bleeding, allowing advanced preparation of blood components. Perioperative monitoring of the clotting function of the patient's blood is also important because coagulopathies can be induced by hemodilution of procoagulants, fibrinogen and platelets, by consumption of coagulation factors during surgical procedures, or by cardiopulmonary bypass. Post operative assessment of clotting function is also crucial to the patient's successful recovery. For example, 3-5% of cardiopulmonary bypass patients require surgical reoperation to stop bleeding. Prompt assessment of clotting function could rule out coagulopathy as the cause of bleeding and could avoid unnecessary surgery that adds to patient morbidity and treatment costs.
Several tests of coagulation are routinely utilized to assess the complicated cascade of events leading to blood clot formation and test for the presence of abnormalities or inhibitors of this process. Among these tests are platelet count (PLT), prothrombin time (PT), partial thromboplastin time (aPTT), activated clotting time (ACT), fibrinogen level (FIB) and fibrinogen degradation product concentrations. The aPTT test can also be used to assess the degree of anticoagulation resulting from heparin administration, while the PT test results can indicate the level of anticoagulation produced by warfarin administration.
During heart bypass surgery, the platelets of blood circulated in an extracorporeal circuit may become activated by contact with the materials present in the extracorporeal circuit. This activation may be reversible or irreversible. Once platelets are irreversibly activated, they lose their ability to function further. A deficiency of functional platelets in the blood may be indicative of an increased probability of a post-operative bleeding problem. Such a deficiency, and the, resulting post-operative bleeding risk, could be remedied by a transfusion of platelet concentrate. Platelet functionality tests, e.g., the ACT test, can identify a deficiency of platelets or functional platelets and aid the attending surgeon in ascertaining when to administer a platelet concentrate transfusion. Such a test is further useful in ascertaining the efficacy of a platelet transfusion. By performing the platelet functionality test following a platelet transfusion, it is possible to determine if additional platelet concentrate transfusions are indicated. Real-time assessment of clotting function at the operative site is preferred to evaluate the result of therapeutic interventions and also to test and optimize, a priori, the treatment choice and dosage.
A number of different medical apparatuses and testing methods have been developed for measuring and determining platelet activation and coagulation-related conditions of blood that can be used in real time during surgery, particularly bypass surgery, on fresh drawn blood samples or that can be used after some delay on citrated blood samples. Some of the more successful techniques of evaluating blood clotting and coagulation of fresh or citrated blood samples employ plunger techniques disclosed in commonly assigned U.S. Pat. Nos. 4,599,219, 4,752,449, 5,174,961, 5,314,826, 5,925,319, and 6,232,127, for example.
As shown in the figures of the '127 patent, for example, these automated instruments employing the plunger technique for measuring and detecting coagulation and coagulation-related activities receives a blood filed syringe and a cartridge. The cartridge includes a plurality of test cells, each of which is defined by a tube-like member having an upper reaction chamber where the analytical test is carried out and a lower reagent chamber that contains a reagent or reagents and/or other compounds as disclosed in the above-referenced commonly assigned patents. For example, the reagents and compounds in at least one of the cells comprise a platelet activation reagent to activate coagulation of the blood in order to determine the ACT.
As disclosed in the above-referenced '127 patent, certain discoveries have been made which contribute to a better understanding of the role of platelets in an ACT test. Such discoveries suggest that the activation of the platelets has a significant and previously unappreciated effect on ACT test results. While it has long been suspected that platelet activation contributes to total blood coagulation times, until fairly recently, there has been no technique available for confirming and quantifying the impact of platelet activation on ACT. The above-referenced '826 patent discloses an improved ACT test that includes a platelet activation phase to accommodate the effects of platelet activation. An activating reagent is mixed with a sample of blood to be tested, and then the mixture is gently agitated in such a manner and for a period of time sufficient to establish a predetermined and predictable contribution to the ACT from platelet activation. Two simultaneous ACT tests (with different platelet activation phases) are performed to evaluate platelet function, and the difference between the resulting ACT tests is indicative of the platelet functionality of the sample of blood. In a further improvement disclosed in the above-referenced '319 patent, the sample of blood is mixed with a chemical platelet activating agent to facilitate the participation of active platelets in the blood clotting reaction, thereby shortening the clotting time of the blood. If the platelets are inactive or not functioning normally, the activator will have minimal or no effect on the clotting time.
More particularly, each cartridge cell is formed by a downwardly tapered, open-ended, tube of transparent glass or plastic material. A resilient, flexible, sliding plug seals the lower end opening of the tube below the reagent chamber. The sliding plug is adapted to be engaged and driven upward into the reagent chamber by a plug driver shaft of the instrument. The tube wall is shaped to define an inwardly projecting annular seat intermediate the upper reaction chamber and the lower reagent chamber. The annular seat defines an upper annular sealing surface and a lower annular sealing surface. Each cartridge cell contains an elongated plunger that comprises an elongated plunger shaft extending between an upper, laterally extending “flag” disposed above the tube upper end opening and a sealing washer or disk (also referred to as a “daisy”) that is initially seated against the upper and lower annular sealing surfaces to seal the reaction chamber from the reagent chamber when a blood sample is dispensed into the reaction chamber. The plunger shaft is disposed in the center of the reaction chamber when the plunger is seated.
The use of the instrument and the cartridge is depicted in FIG. 5 of the above-referenced '127 patent. A syringe filled with blood is manually inserted into a syringe receptacle of the instrument. The cartridge is manually inserted into a cartridge receptacle of the automated coagulation timer instrument. Discrete blood samples are automatically dispensed from the syringe into the upper reaction chambers of the cells. When the test commences, an actuator of the instrument engages all of the flags of the plunger assemblies in the cells of the cartridge and lifts the plunger assemblies to unseat the respective sealing disks. At the same time, the plug driver shafts are driven upward against the plugs to move the plugs upward and force the contents of the reagent chambers through the seat opening into the reaction chambers to be mixed with the blood samples. The plunger assemblies are moved up and down one or more times to mix the blood samples and reagent. The plunger flags are lifted to a starting position and released by the actuator. The plunger assembly descends by the force of gravity, resisted by the viscosity of the blood in the reaction chamber, until the sealing disk either contacts the upper annular sealing surface or is halted by contact with a blood clot that forms in the reaction chamber above the upper annular sealing surface.
The movement of the flag of the plunger assembly is photo-optically tracked by the instrument. The instrument detects and times out the movement of the plunger assembly and the point at which it stops descending in a manner disclosed in the above-referenced '127, '219, and '319 patents. The coagulation-related activity is detected upon a sufficient change in the descent rate and indicated by the instrument. In particular, the ACT of the blood in each cell of the cartridge is timed out, displayed, and stored in memory, and the cartridge array is withdrawn from the cartridge receptacle.
A less expensive and simplified, ACT II® automatic coagulation timer, is commercially sold by the assignee of this patent application that receives a cartridge having two cells of the type described above that are already filled with blood by a user as described below. The ACT II® instrument does not include the receptacle for the blood filled syringe and the automatic blood dispenser for moving the syringe over each upper cell opening and ejecting the blood sample from the syringe.
In use of the simplified ACT II® instrument to determine coagulation time of a whole blood sample or plasma in an operative procedure, the user typically draws the patient's whole blood or plasma into a syringe and then manually dispenses the blood samples into the upper reaction chambers of the two cartridge cells. For samples that are citrated, the use of a precision pipettor and pipette tips can alternatively be used. It is important that the amount of blood dispensed into each reaction chamber be relatively equal and sufficient in volume without over-filling the reaction chamber to accurately perform the ACT tests and avoid contamination of the instrument. Thus, the user must carefully judge and visually observe the amount of blood ejected from the syringe or pipettor into the reaction chamber.
The blood must dispensed deeply into the reaction chamber to avoid depositing blood droplets on the flag or on the plunger shaft above the upper level of the blood sample that would tend to weight the plunger and contaminate the cartridge receptacle of the instrument. Thus, the user must take care to properly deposit the blood sample into the reaction chamber of each cell.
The flag must be manually deflected to one side of the cell without breaking the seal between the upper reaction chamber and the lower reagent chamber to insert the needle or pipette tip into the upper open end. Therefore, the user typically grasps the cartridge and pushes the flag aside with a gloved finger when the needle tip or pipette tip is inserted through the upper open end. The syringe needle tips are sharp, and there is a possibility of a needle puncture of the user's finger or hand when holding the cartridge steady and upright and diverting the flag aside to insert the needle tip into the upper open end.
Thus, although previous instruments using the plunger sensing technique have proven generally satisfactory, the need for certain enhancements has been identified.